In general, the surface of a solid base material such as a metal or polymer has a specific surface energy. The specific surface energy appears as a contact angle between a liquid and a solid when the liquid contacts the solid. Here, the liquid generally refers to water or oil, etc., and water shall be representatively mentioned as the liquid hereinafter. If the contact angle is smaller than 90° a spherical water drop loses its form on the surface of the solid to wet the surface of the solid, exhibiting wettability. If, however, the contact angle is larger than 90° the spherical water drop maintains its form on the surface of the solid to easily flow by an external force rather than wet the surface of the solid, exhibiting non-wettability. For example, if a water drop falls on a lotus leaf, it does not wet the lotus leaf but flows on the surface of the lotus leaf. This phenomenon indicates non-wettability.
The value of the specific contact angle of the surface of the solid base material may change if the surface is processed to have fine protrusions and depressions (unevenness). Namely, a hydrophilic surface having a contact angle of smaller than 90° can have greater wettability through surface processing, and a hydrophobic surface having a contact angle of larger than 90° can have a greater non-wettability through surface processing.
A technique for changing the contact angle of the surface of the solid for is known, to date, as a MEMS (Micro Electro Mechanical Systems) process to which a semiconductor fabrication technique is applied. However, the MEMS process, an up-to-date technique which applies the semiconductor technique thereto by mechanical engineering, is disadvantageous in that its fabrication process is very complicated as well as incurring much fabrication cost. Namely, in order to form the protrusions and depressions of a nano-scale on the surface of the solid by using the MEMS process, operations such as oxidizing the metal surface, applying certain temperatures and certain voltages, and oxidizing and etching in a special solution are performed. The MEMS process cannot be performed in a general working environment but should be performed in a specially fabricated clean room, and machines required for the operation are high-priced equipment. Thus, the MEMS process for forming the hydrophobic surface can hardly be employed because its process is very complicated, it is not suitable for mass production, and it incurs high fabrication costs.
In addition to the MEMS process technique, there is a technique for changing the contact angle on the surface of the solid by using a chemical processing method. However, a membrane fabricated according to the chemical processing method exhibits a low bonding force with a base material and has a coating film that is easily damaged by friction. In addition, the chemical processing method makes it difficult to chemically process only a certain particular region, so it is disadvantageous in that hydrophobicity and hydrophilicity cannot be provided together or hydrophobicity cannot be provided only to a certain region.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.